Enjoyed the episode? Want to listen later? Subscribe here, or anywhere you get podcasts:

I think these people weren’t less intelligent than us, not at all, but I guess one of the fundamental lessons from all this is that background assumptions really matter. Not just on what might seem like an atomistic claim about the way the world is, but also what we should do in it.

Tom Moynihan

It can be tough to get people to truly care about reducing existential risks today. But spare a thought for the longtermist of the 17th century: they were surrounded by people who thought extinction was literally impossible.

Today’s guest Tom Moynihan, intellectual historian and author of the book X-Risk: How Humanity Discovered Its Own Extinction, says that until the 18th century, almost everyone — including early atheists — couldn’t imagine that humanity or life could simply disappear because of an act of nature.

This is largely because of the prevalence of the ‘principle of plenitude’, which Tom defines as saying:

Whatever can happen will happen. In its stronger form it says whatever can happen will happen reliably and recurrently. And in its strongest form it says that all that can happen is happening right now. And that’s the way things will be forever.

This has the implication that if humanity ever disappeared for some reason, then it would have to reappear. So why would you ever worry about extinction?

Here are 4 more commonly held beliefs from generations past that Tom shares in the interview:

  • All regions of matter that can be populated will be populated: In other words, there are aliens on every planet, because it would be a massive waste of real estate if all of them were just inorganic masses, where nothing interesting was going on. This also led to the idea that if you dug deep into the Earth, you’d potentially find thriving societies.
  • Aliens were human-like, and shared the same values as us: they would have the same moral beliefs, and the same aesthetic beliefs. The idea that aliens might be very different from us only arrived in the 20th century.
  • Fossils were rocks that had gotten a bit too big for their britches and were trying to act like animals: they couldn’t actually move, so becoming an imprint of an animal was the next best thing.
  • All future generations were contained in miniature form, Russian-doll style, in the sperm of the first man: preformation was the idea that within the ovule or the sperm of an animal is contained its offspring in miniature form, and the French philosopher Malebranche said, well, if one is contained in the other one, then surely that goes on forever.

And here are another three that weren’t held widely, but were proposed by scholars and taken seriously:

  • Life preceded the existence of rocks: Living things, like germs or microorganisms, came first, and they extruded the earth.
  • No idea can be wrong: Nothing we can say about the world is wrong in a strong sense, because at some point in the future or the past, it has been true.
  • Maybe we were living before the Trojan War: Aristotle said that we might actually be living before Troy, because it — like every other event — will repeat at some future date. And he said that actually, the set of possibilities might be so narrow that it might be safer to say that we actually live before Troy.

But Tom tries to be magnanimous when faced with these incredibly misguided worldviews.

I think that something almost similar to scope neglect can happen, where we see the sheer extent of ignorance in the past and therefore think that is boundless. And this could lead you to think therefore our progress is also made insignificant within this boundless sea, but no, I think it’s structured. There are bounds to ignorance and we’re making progress, but within a space that’s potentially far bigger than we can currently think of.

In this nearly four-hour long interview, Tom and Rob cover all of these ideas, as well as:

  • How we know the ancients really believed such things
  • How we should respond to wacky old ideas
  • How we moved on from these theories
  • How future intellectual historians might view our beliefs today
  • The distinction between ‘apocalypse’ and ‘extinction’
  • The history of probability
  • Utopias and dystopias
  • Big ideas that haven’t flowed through into all relevant fields yet
  • Intellectual history as a possible high-impact career
  • And much more

Get this episode by subscribing to our podcast on the world’s most pressing problems and how to solve them: type 80,000 Hours into your podcasting app. Or read the transcript below.

Producer: Keiran Harris
Audio mastering: Ben Cordell
Transcriptions: Sofia Davis-Fogel


Principle of Plenitude

Tom Moynihan: So the principle of plenitude directly stated says that whatever can happen will happen. In its stronger form it says whatever can happen will happen reliably and recurrently. And in its strongest form it says that all that can happen is happening right now. And that’s the way things will be forever.

Rob Wiblin: So this principle of plenitude as you’re laying it out would have the implication that if humanity ever disappeared for some reason, then it would have to reappear, because the universe should be full of valuable things, like not wasting an opportunity to create something valuable. Is that basically right?

Tom Moynihan: Yeah. So I think that the way the principle is most commonly articulated, particularly within the Christian tradition, comes with a load of value claims, lots of axiology. So the idea is often that the world is as maximumly full of valuable things as it can be. But you don’t even need those value claims to arrive at a idea of the imperishability or indestructibility of species, but lots of pre-Christian people, the pagan worldview, a lot of them talk about humanity and its role within creation quite indifferently, but they still believed in plenitude invariably. And that led them to this idea that humanity would return, regardless of how valuable you might think it is, how central its role is in the universe. Because again, if you’re yielding the definition of possibility based on frequency within time, that means that your only definition of possibility is something is possible if it sometimes happens. Now that sometimes goes forwards in time and backwards in time. So if humanity is something happening currently right here, right now, we can reliably say that it will be happening again at some arbitrarily far future date, regardless of if there’s some arbitrarily long interval in time where humanity isn’t happening. So it creates this sense of eternal return.

How do we know they really believed this?

Tom Moynihan: Yeah. So people do say it quite explicitly. So Aristotle for example, in [Decalur], he says, “Anything then which always exists is absolutely imperishable.” I find it really interesting that he says, what the train of thought is, to unpack that, is that he’s saying that if it was possible for species to go extinct and never return, that would have already happened, because what is possible is what sometimes has been known to happen. But we find ourselves in a world filled with species, and filled with all kinds of things. So therefore these things must be imperishable. So he’s there yielding the definition of omnitemporal, or always known to exist, in terms of necessity. So it’s necessitous that species are the way they are and that they continue existing. People continue to make this claim throughout the whole tradition up until some point in the 1800s.

Rob Wiblin: Do you have any other quotes from people… We’ve talked most about the ancient Greeks here, but I guess it seems as though you’re saying even natural scientists were appealing to this principle as late as the 1800s, although I guess it started to wane around then.

Tom Moynihan: Yeah. So I’d love to read out some good examples. Fast forwarding to the Romans, but returning to Lucretius who I mentioned earlier, he’s an interesting case because there were some points in his poem De rerum natura where he puts forward his Epicurean worldview, and it’s quite atheistic. It’s very materialistic. And there are also points in it where he says that the world itself is aging. And one day the world will perish. It will fall apart. He also talks about species that seem to not exist anymore. He talks about them in the sense that they are abominations, or monstrous creations that had to be weeded out of creation rather than irreversibly disappeared, viable species.

Tom Moynihan: So he says these things that seem very modern. And people have read into this, that Lucretius was an early proponent of extinctions and irreversibility within nature. But then he also says this, and I quote, he says, “It is impossible for anything to return to nothing. And so no visible object ever suffers total destruction. Since nature renews one thing from another, there is the further consideration that, in the totality of created things, there is nothing solitary, nothing that is born unique and grows unique and singular. Therefore, you can have no final instance of a dying kind.” And then he also says, and this will become relevant later on I think, he says, “You are bound to admit that in other parts of the universe, there must be other worlds inhabited by many different peoples…” (note peoples) “…and species of wild beasts.” So those are all plenitude-derived assumptions, and I think that’s a very good example. Because Lucretius elsewhere says these things that can lead you into thinking that he is thinking in a way that we’d recognize as modern about extinction.

Rob Wiblin: Yeah. I’m slightly asking for these quotes because I can imagine people in the audience being skeptical about this, as I was initially skeptical when I heard this claim. It just sounds so fabulous and such a strange view for people to have almost, I didn’t quite… And it’s so convenient for me to think oh, past generations had this wacky view and that’s why they didn’t share my concern about the risk of extinction. But yeah, it does seem like the accumulation of quotes that you have in the book did eventually convince me that, at least as far as I can tell, you’re on the right track. What about a quote from someone more recently, in Europe?

Tom Moynihan: I have a couple that I think might be useful. So a quick one from Descartes… Actually this one’s rather interesting. He says, “Due to the laws of nature, matter takes on successively all the forms of which it is capable. Therefore, if we consider these forms in order, we could eventually arrive at the one which is our present world. So that in this respect, no false hypothesis can lead us into error.”

Rob Wiblin: Can you translate that?

Tom Moynihan: Yeah, we’ll begin to talk about this later on, unpack it later, this idea that plenitude or infinities, eternities, can create interesting practical consequences. But here’s also theoretical consequences of thinking in terms of plenitudes. Not to do with morality, but to do with the epistemology. Descartes is saying that if plenitude is true and everything that can happen will happen, then no false hypothesis, no idea that we can say about the world is wrong in some strong sense, because at some point in the future or the past, it has been true. It’s a really interesting claim. And yeah, again, hopefully later on, we can unpack how that converts from doxastic to normative ideas, but yeah, I think that’s quite a nice and strong example.

How to react to wacky old ideas

Tom Moynihan: My policy when it comes to reading this is always to have a kind of attitude of magnanimity to these incredibly wrong— by our lights — worldviews. I think that something almost similar to scope neglect can happen, where we see the sheer extent of ignorance in the past and therefore think that is boundless. And this could lead you to a position where you think therefore our progress is also made insignificant within this boundless sea, but no, I think it’s structured. There are bounds to ignorance and we’re making progress, but within a space that’s potentially far bigger than we can currently think of.

Tom Moynihan: I also think another point I’d want to make is that we all take for granted these ideas that are incredibly important to our thinking about the world. I think lots of ideas are crucial considerations, and we take them for granted. The way that I like to think of it is we often take for granted our skeleton, or our functional breathing. We don’t have to think about those kinds of somatic realities, but nonetheless, they underwrite every single thing that we’re capable of doing. Obviously if we sat there in awe all the time of all the contributions of scientists, naturalists, and philosophers, we wouldn’t be able to get anything done. But nonetheless, I think that they do deserve that, because these people in the past might’ve been as intelligent as us in the same way that some counterfactual human that has no skeleton but the same muscle mass as one of us does, but in the same way that counterfactual boneless person can’t do anything, you require the backbone of all these previous thoughts to actually enable us… Yeah, I guess what I’m saying is that we all do stand on the shoulders of giants.

How we got past these theories

Tom Moynihan: So the gap in the solar system, the Mars-Jupiter asteroid belt, turns out that it’s not a ruined planet, but that was a hypothesis for a long period of time. And yes, it created the sense of there being this huge vacuity in our kind of cosmic neighborhood. So William Godwin, Mary Wollstonecraft‘s husband, Mary Shelley‘s dad, was really disturbed by this. He said God does not suffer any region of matter to be unpopulated, but here we have this planetary ruin on our doorstep. All of these insights build up and build up. And it took a very long time to trickle into the consciousness of the scientific community. But this idea that the earth was unliving for a long period of time, obviously now we’ve actually updated that and we know that life has been around on Earth pretty much since the beginning, but it’s just been very unicellular uncomplex life. But this idea that there was nothing interesting, nothing complex going on on the planet for a long period of time, you find that in the very beginning of geology with Nicholas Steno, but it begins to trickle very slowly into consciousness around kind of the 1800s.

Tom Moynihan: It begins to pick up and you get this beginning of the forcing of the acknowledgement of there being genuine historicity in the planet. Why that’s important to what we’re talking about — the long-term future and what can be achieved within it — is that this puts a very important end stop on previous eternities wherein everything achievable has already been achieved. So in the 1800s, people began to think that intelligence is in a sense, a recent potential phenomenon on the planet. Therefore, all its potentials haven’t already been exhausted. So I think it’s really important, that backstop on previous eternity. Now, obviously it’s indexical because it’s on this planet. So you still have all those other planets in which everything achievable has been achieved and people continue to think that off until well, into the late 1800s, potentially even later. But yeah, that’s a very important discovery.

Intellectual history

Tom Moynihan: I would use a comparison with paleontology. Paleontologists do not have access to the whole fossil record, because plenty of animals — probably the vast majority of animals — aren’t recorded. It’s the same way. The vast majority of human thoughts aren’t recorded. Some of them are, and some of them are passed down to us.

Tom Moynihan: But nonetheless, even though you have very incomplete records, you still get a sense of the major shape of life, if we’re talking paleontology, but also in terms of the record of ideas, I think you do get that. You know the overall shape and you can pinpoint novelties and emergences within it. So there’s this thing in paleontology, the Signor-Lipps effect, which is a biasing effect where, well it’s an attempt to account for a bias, where it’s argued that you should never consider any actual fossil the first or the last instance in the actual record, because of the incompleteness of the record.

Tom Moynihan:I try to think about and apply a similar principle when I’m thinking about how all of this fits together. Sure, I’m not going to ever probably get the first instance of a new idea. I think actually in some cases you can, The Last Man is the first instance of a novel about human extinction in a modern sense. But with these more substructure logical background assumptions, yeah. I think it’s hard. And you do have to take that into account. But nonetheless, yeah. The biologist J.B.S. Haldane was once asked what would knock his confidence in the theory of evolution, and he answered “a Precambrian rabbit.” I would answer the same question about the very basic rudimental shape of what I’ve tried to tell when it comes to the Western tradition. The equivalent would be if someone could point me to someone writing in 300 BC that had anything as insightful or detailed to say about the future of humanity as, say, Toby Ord. That’s the Precambrian rabbit. I know that’s setting the goalposts very wide, but I do think that even though the record is very incomplete and my reading of it is very incomplete you still can see the basic structure, particularly when it comes to important ideas.

Apocalypse vs. extinction

Tom Moynihan: I think that in religious and mythological traditions, the idea of the apocalypse is imbued with a lot of significance. So what I mean by that is that it’s often seen as the culmination of the moral order, particularly within the Christian, the Abrahamic religions. It’s seen as this vast culmination, conclusion of what is moral. So in rapture, God sorts the good souls from the bad souls. Even though it’s probably inscrutable to us mortal souls, nonetheless, the ultimate good is still consummated. And that means that nothing’s at stake in our individual actions, because we might be a bad person, that might be bad in some very local sense, but nonetheless, in the end that big sorting is going to happen, so nothing’s really at stake. Whereas, in extinction, in the modern idea of extinction, if we’re believing that in some meaningful sense, there aren’t any other intelligent agents in our cosmic vicinity, it’s this sense of this irreversible terminal frustration of what we think of as the moral order.

Tom Moynihan: So be that the impartial demand to make the world a better place, not just from moral agents, but also from moral patients: animals, other sentiences… It’s the frustration of that upward force, that escalator to betterness, which you just don’t find in the apocalypse. And there are a couple of other things I mentioned earlier: the idea of the physical world continuing without that agency around. In apocalypses, it’s the afterlife. Everything just switches into this land beyond. So even though there are some exceptions, like I mentioned earlier, these other traditions outside of the Christian one that have some of these factors in them, they don’t have all of them. And so the way I sum it up is apocalypse secures a sense of an ending, whereas extinction anticipates the ending of sense.

Tom Moynihan: So it’s a very different outlook. Whereas in the apocalypse, nothing’s at stake. Potentially in the possibility of extinction, in a sense, everything is at stake. I’ve changed the way I talk about this in the book. I think I imbued too much unique significance on humanity. But again, everything is at stake in that sense of there being a kind of direction and that morally meaningful universe.

Are we making progress?

Tom Moynihan: So I began, this is around a decade ago, I began having that prior worldview, the kind of post-modernist, deconstructionist one, post-humanist is the current vogue in lots of continental theory. And that’s just because of the corner of the humanities that I emerged from. Instead of being taught Quine, I was taught Derrida. So I started off with that kind of worldview. And then it was from trying to look at the Enlightenment, trying to dismantle it, that requires actually reading the Enlightenment and I was actually very convinced by a lot of its texts. And also just through reading through… I was just convinced that there are these vast gains in insight and also moral insight, more importantly, that have happened. And I think it’s just undeniable that humanity has gained a lot more insight when it comes to the big picture.

Tom Moynihan: So things like the fact that you go back a certain amount of time and everyone thinks that the universe is in its most optimal, valuable state. William Paley, the famous argument from design theologian, this was around 1800, he said we might not like mosquitoes and mice, but they’re actually really good because when an area is unpopulated by humans, it will be filled by these lower creatures that are kind of placeholder vehicles of sentience that mean that value in the world can never dip below some certain level.

Tom Moynihan: No one would say that anymore, or at least not many people would say that. Or for example, Alexander Pope in his brilliant long poem, The Essay on Man, he says that everything that happens in the universe happens for a good end, including the destruction of Earth. And it’s good that we just don’t know when that will happen, because otherwise we’d worry about it. And he compares this to the lamb that licks the hand of the slaughterer. That’s barbaric, right? So the fact that we now wouldn’t say those things, I think is just in my view, undeniably a gain in insight. So I think actually picking through the history of ideas, and this is me talking from my own personal experience, so take it with a pinch of salt, but it actually converted me away from a pessimistic worldview towards one that’s more convinced in progress or at least the possibility of progress.

Articles, books, and other media discussed in the show

Tom’s work


Papers and essays

Other media

Related episodes

About the show

The 80,000 Hours Podcast features unusually in-depth conversations about the world's most pressing problems and how you can use your career to solve them. We invite guests pursuing a wide range of career paths — from academics and activists to entrepreneurs and policymakers — to analyse the case for and against working on different issues and which approaches are best for solving them.

The 80,000 Hours Podcast is produced and edited by Keiran Harris. Get in touch with feedback or guest suggestions by emailing [email protected].

What should I listen to first?

We've carefully selected 10 episodes we think it could make sense to listen to first, on a separate podcast feed:

Check out 'Effective Altruism: An Introduction'

Subscribe here, or anywhere you get podcasts:

If you're new, see the podcast homepage for ideas on where to start, or browse our full episode archive.